[squidtone]

JT,
I just got done today putting my Muncie M20 back together. (bad gears, got a whole new set of Autogear M22 "Rock crusher" wide ratio gears) Here are some things I learned while doing this whole job:

Your slider is correct. The bevels on the sliders face away from each other, so the 1st-2nd slider bevel faces toward the back.

Muncies are infamous for the reverse idler coming into contact with the slider when in first gear. Some folks actually make a larger bevel on each tooth on the reverse idler gear with a bench grinder. That may be drastic....

The rotation of the countergear should always tend to force the idler gear toward the back of the case. While you do have some endplay on that idler gear, is the reverse idler shaft able to seat all the way into the tail housing? Is there gunk in the shaft bore or roll pin slots preventing the shaft from moving back far enough for that extra few thousandths of clearance? If the shaft can go back a wee bit more, it may be enough for clearance.

the other thing that occurs to me is that maybe the slider doesn't have to move all the way against the helical teeth...it only needs to engage the engagement teeth...can you adjust your shifter stops (if you have them)?

Dave